1. Field of the Invention
The present invention generally relates to a method and apparatus for manufacturing solder balls and, more particularly, to a method and apparatus for manufacturing solder balls for BGA (Ball Grid Array) used recently for electronic packaging.
2. Description of the Related Art
As is well known to those skilled in the art, conventional methods for manufacturing tiny metal balls may be classified into three types: a centrifugal spraying type method, a gas-related type method and a method wherein a metal piece having a uniform weight is molten and forms many balls in liquid and the balls are solidified again. However, with the centrifugal spraying type method and the gas-related type method, ball size is not uniform due to their manufacturing characteristics. Also, a very large apparatus is required for solidifying the balls in gas because the balls are greatly accelerated in the manufacturing process. On the other hand, according to the method wherein a metal piece having a uniform weight is molten and forms many balls in liquid and the balls are solidified again, many processes are required prior to the solidification because metal pieces having uniform weights must be made, thus increasing manufacturing cost. In brief, the conventional methods have low gaining or yield rates and are not economical.
There is another method wherein molten metal flows downward through a small orifice. This method uses a natural phenomenon in which liquid in a fine flow is in an unstable state due to boundary surface tension and the liquid is easily cut and forms many droplets by slight waves on a flowing surface. Lord Rayleigh analyzed this phenomenon and explained that the wavelength of the most unstable wave is about 1.9 times as large as the diameter of the orifice in an article xe2x80x9cOn the Instability of Jetsxe2x80x9d, London Mathematical Society, 1878. Accordingly, the diameters of the generated droplets have a normal distribution centering at the diameter about 1.9 times as large as the diameter of the orifice. In such a case, when a vibration having a uniform frequency is applied to the liquid, waves having a uniform periodic time are generated on the surface of the liquid passing through the orifice and the waves are cut into fractions having a uniform size, thereby forming many metal balls.
Such a technique using the natural phenomenon is disclosed in xe2x80x9cInkjet Printer of IBM, J, Resent Developmentxe2x80x9d published in 1974, the method of vibrating a disc in molten metal of U.S. Pat. No. 5,266,098 and the method of applying vibrations from the exterior of liquid to the liquid using a speaker of xe2x80x9cPower and Powder metallurgyxe2x80x9d, Vol. 38, No. 6 published in Japan in 1991. However, this method requires a large apparatus because small droplets are easily solidified in the atmosphere, but large droplets need a long falling distance so as to be solidified, may not regulate the size of balls and reduces a gaining rate.
In order to overcome this defect, the method of solidifying droplets using cooling liquid is proposed in xe2x80x9cThe International Journal of Power Metallurgyxe2x80x9d, Vol. 32 published in 1996. However, according to this method, since molten metal is boiled at the surface of a droplet upon a droplet falling into cooling liquid, the shape of the ball is determined in accordance with the boundary surface tension and the size of the molten metal, thus deteriorating the surface quality and the degree of sphericity of ball.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art and an object of the present invention is to provide a method and apparatus for manufacturing solder balls, reducing manufacturing cost due to their simple processes and increasing a production yield or gaining rate due to the uniform size of balls.
In order to accomplish the above object, the present invention provides a method of manufacturing solder balls comprising the step of dividing molten metal into droplets of a uniform size by applying vibrations to the molten metal stream and the step of sphering the droplets by dropping the droplets into cooling liquid that has a temperature gradient ranging from a temperature higher than a melting point of the metal to a normal temperature.
According to another embodiment of the present invention, the cooling liquid may have a boiling point higher than a melting point of the metal, a relatively low density and a relatively low coefficient of viscosity and is not easily reacted with the metal at a normal temperature.
In addition, the present invention provides an apparatus for manufacturing solder balls, comprising a tundish having one or more orifices at its bottom and containing molten metal controllable in its level, a vibrator being immersed in the molten metal of the tundish and generating vibrations, a cooling liquid tank being situated under the tundish and being provided with a cooling liquid heater at its upper and middle outer surface, a cooling liquid cooler at its lower outer surface and a cooling liquid discharge conduit at its top, an inactive atmospheric chamber being interposed between the bottom of the tundish and the top surface of the molten metal, a molten metal receiving tray being seated on a portion of the inactive atmospheric chamber and being horizontally movable, a ball colleting barrel being positioned under the cooling liquid tank and being provided with a cutoff valve at its top, a ball removing valve at its bottom and a cooling liquid supply conduit at its upper portion, and a cooling liquid reservoir being situated under the ball collecting barrel, being provided with a ball collecting sieve at its inlet and being connected with the cooling liquid discharge conduit and the cooling liquid supply conduit.
According to another embodiment, the orifice may be configured to be diminished from its top to its bottom so as to transmit vibrations generated by said vibrator to the stream flowing through the orifice.